The CIA left “explosive training material” under the hood of a Loudoun County school bus after a training exercise last week, a bus that was used to ferry elementary and high school students to and from school on Monday and Tuesday with the material still sitting in the engine compartment, according to the CIA and Loudoun County officials.

The Loudoun County Sheriff’s Office and the CIA said in statements Thursday that the explosive material was left behind after a training exercise at Briar Woods High School during spring break. The CIA said it was a training scenario for explosives-detecting dogs.

CIA officials said in a statement that the material “did not pose a danger to passengers on the bus,” which was used on March 28 and 29. Authorities held a joint training program at Briar Woods from March 21 to 24.

Loudoun schools spokesman Wayde Byard said the CIA indicated the nature of the material but asked the school system not to disclose it. Byard described it as a “putty-type” material designed for use on the battlefield and which requires a special detonator; such putty, or plastic, explosives — including the well-known C-4 — are used in demolition and are considered stable.

Byard said law enforcement agencies use school facilities on occasion to conduct realistic training exercises, including active-shooter drills.

As part of last week’s training exercise, CIA trainers placed explosive material into the engine compartment of a school bus on Thursday to test a dog’s ability to sniff it out. They also placed the material in parts of the school. Byard said the dog successfully found the material in the engine compartment, but some of the material fell deeper inside the compartment and became wedged beneath the hoses.

He said school bus drivers check under the hoods of their buses before they take them out on the road, but the package was wedged too far deep inside the engine compartment and was the same color as the hoses, so it could not easily be seen.

The bus shuttled students to and from school for two days with the explosive material under the hood, making eight runs totaling 145 miles and carrying 26 students attending Rock Ridge High School, Buffalo Trail Elementary School and Pinebrook Elementary School.

The bus was taken to a school system facility on Wednesday for routine maintenance. Byard said the county’s buses are regularly taken off-line to check their spark plugs, hoses and to rotate tires. It was during a routine inspection that a technician discovered the explosive material.

The school system immediately notified the county sheriff’s office and the fire marshal, who removed it. The CIA also helped remove the material.

“The training materials used in the exercises are incredibly stable and according to the CIA and Loudoun County explosive experts the students on the bus were not in any danger from the training material,” according to a Sheriff’s Office statement.

Officials said they checked all other buses at the school as a precaution.

School officials on Thursday met with the CIA, Loudoun County Sheriff’s Office, the fire marshal and county administrators and determined that all law enforcement training exercises at schools would be suspended until stronger protocols are established.

“We’re all very upset by what happened, but we’re going to review everything that did happen,” Byard said. “Obviously we’re concerned. The CIA really expressed its deep concern and regret today, and it was sincere.”

CIA officials acknowledged the error in a statement and confirmed that an agency canine unit ran a training exercise with local agencies last week in the county. Agency officials said they were notified by Loudoun officials Wednesday and coordinated with them to recover the material.

The intelligence service said that both CIA and Loudoun County experts said the explosive material did not pose a danger to passengers on the bus. The agency statement said they would take “immediate steps to strengthen inventory and control procedures in its K-9 program” and that they will investigate the canine training program.

The CIA said in the statement that the agency accounted for all training explosives after performing a full inventory Thursday.